2,312 research outputs found
Les qualités souhaitables dans des systèmes statistiques en se servant de GENSTAT comme référence
TDIR: Time-Delay Interferometric Ranging for Space-Borne Gravitational-Wave Detectors
Space-borne interferometric gravitational-wave detectors, sensitive in the
low-frequency (mHz) band, will fly in the next decade. In these detectors, the
spacecraft-to-spacecraft light-travel times will necessarily be unequal and
time-varying, and (because of aberration) will have different values on up- and
down-links. In such unequal-armlength interferometers, laser phase noise will
be canceled by taking linear combinations of the laser-phase observables
measured between pairs of spacecraft, appropriately time-shifted by the light
propagation times along the corresponding arms. This procedure, known as
time-delay interferometry (TDI), requires an accurate knowledge of the
light-time delays as functions of time. Here we propose a high-accuracy
technique to estimate these time delays and study its use in the context of the
Laser Interferometer Space Antenna (LISA) mission. We refer to this ranging
technique, which relies on the TDI combinations themselves, as Time-Delay
Interferometric Ranging (TDIR). For every TDI combination, we show that, by
minimizing the rms power in that combination (averaged over integration times
s) with respect to the time-delay parameters, we obtain estimates
of the time delays accurate enough to cancel laser noise to a level well below
the secondary noises. Thus TDIR allows the implementation of TDI without the
use of dedicated inter-spacecraft ranging systems, with a potential
simplification of the LISA design. In this paper we define the TDIR procedure
formally, and we characterize its expected performance via simulations with the
\textit{Synthetic LISA} software package.Comment: 5 pages, 2 figure
LISA Science Results in the Presence of Data Disturbances
Each spacecraft in the Laser Interferometer Space Antenna houses a proof mass
which follows a geodesic through spacetime. Disturbances which change the proof
mass position, momentum, and/or acceleration will appear in the LISA data
stream as additive quadratic functions. These data disturbances inhibit signal
extraction and must be removed. In this paper we discuss the identification and
fitting of monochromatic signals in the data set in the presence of data
disturbances. We also present a preliminary analysis of the extent of science
result limitations with respect to the frequency of data disturbances
An Estimation of the Gamma-Ray Burst Afterglow Apparent Optical Brightness Distribution Function
By using recent publicly available observational data obtained in conjunction
with the NASA Swift gamma-ray burst mission and a novel data analysis
technique, we have been able to make some rough estimates of the GRB afterglow
apparent optical brightness distribution function. The results suggest that 71%
of all burst afterglows have optical magnitudes with mR < 22.1 at 1000 seconds
after the burst onset, the dimmest detected object in the data sample. There is
a strong indication that the apparent optical magnitude distribution function
peaks at mR ~ 19.5. Such estimates may prove useful in guiding future plans to
improve GRB counterpart observation programs. The employed numerical techniques
might find application in a variety of other data analysis problems in which
the intrinsic distributions must be inferred from a heterogeneous sample.Comment: 15 pages including 2 tables and 7 figures, accepted for publication
in Ap
Defect-Mediated Emulsification in Two Dimensions
We consider two dimensional dispersions of droplets of isotropic phase in a
liquid with an XY-like order parameter, tilt, nematic, and hexatic symmetries
being included. Strong anchoring boundary conditions are assumed. Textures for
a single droplet and a pair of droplets are calculated and a universal
droplet-droplet pair potential is obtained. The interaction of dispersed
droplets via the ordered phase is attractive at large distances and repulsive
at short distances, which results in a well defined preferred separation for
two droplets and topological stabilization of the emulsion. This interaction
also drives self-assembly into chains. Preferred separations and energy
barriers to coalescence are calculated, and effects of thermal fluctuations and
film thickness are discussed.Comment: revtex4, 13 pages, 12 figure
A Deep Multicolor Survey V: The M Dwarf Luminosity Function
We present a study of M dwarfs discovered in a large area, multicolor survey.
We employ a combination of morphological and color criteria to select M dwarfs
to a limiting magnitude in V of 22, the deepest such ground-based survey for M
dwarfs to date. We solve for the vertical disk stellar density law and use the
resulting parameters to derive the M dwarf luminosity and mass functions from
this sample. We find the stellar luminosity function peaks at M_V = 12 and
declines thereafter. Our derived mass function for stars with M < 0.6 M_sun is
inconsistent with a Salpeter function at the 3 sigma level; instead, we find
the mass function is relatively flat for 0.6 M_sun > M > 0.1 M_sun.Comment: Accepted for publication in AJ. 19 pages including 4 embedded
postscript figures (AASTEX
Point Source Extraction with MOPEX
MOPEX (MOsaicking and Point source EXtraction) is a package developed at the
Spitzer Science Center for astronomical image processing. We report on the
point source extraction capabilities of MOPEX. Point source extraction is
implemented as a two step process: point source detection and profile fitting.
Non-linear matched filtering of input images can be performed optionally to
increase the signal-to-noise ratio and improve detection of faint point
sources. Point Response Function (PRF) fitting of point sources produces the
final point source list which includes the fluxes and improved positions of the
point sources, along with other parameters characterizing the fit. Passive and
active deblending allows for successful fitting of confused point sources.
Aperture photometry can also be computed for every extracted point source for
an unlimited number of aperture sizes. PRF is estimated directly from the input
images. Implementation of efficient methods of background and noise estimation,
and modified Simplex algorithm contribute to the computational efficiency of
MOPEX. The package is implemented as a loosely connected set of perl scripts,
where each script runs a number of modules written in C/C++. Input parameter
setting is done through namelists, ASCII configuration files. We present
applications of point source extraction to the mosaic images taken at 24 and 70
micron with the Multiband Imaging Photometer (MIPS) as part of the Spitzer
extragalactic First Look Survey and to a Digital Sky Survey image. Completeness
and reliability of point source extraction is computed using simulated data.Comment: 20 pages, 13 Postscript figures, accepted for publication in PAS
Improved Stack-Slide Searches for Gravitational-Wave Pulsars
We formulate and optimize a computational search strategy for detecting
gravitational waves from isolated, previously-unknown neutron stars (that is,
neutron stars with unknown sky positions, spin frequencies, and spin-down
parameters). It is well known that fully coherent searches over the relevant
parameter-space volumes are not computationally feasible, and so more
computationally efficient methods are called for. The first step in this
direction was taken by Brady & Creighton (2000), who proposed and optimized a
two-stage, stack-slide search algorithm. We generalize and otherwise improve
upon the Brady-Creighton scheme in several ways. Like Brady & Creighton, we
consider a stack-slide scheme, but here with an arbitrary number of
semi-coherent stages and with a coherent follow-up stage at the end. We find
that searches with three semi-coherent stages are significantly more efficient
than two-stage searches (requiring about 2-5 times less computational power for
the same sensitivity) and are only slightly less efficient than searches with
four or more stages. We calculate the signal-to-noise ratio required for
detection, as a function of computing power and neutron star spin-down-age,
using our optimized searches.Comment: 19 pages, 7 figures, RevTeX
The Lowest Mass White Dwarf
Extremely low mass white dwarfs are very rare objects likely formed in
compact binary systems. We present MMT optical spectroscopy of 42 low mass
white dwarf candidates serendipitously discovered in a survey for hypervelocity
B-type stars. One of these objects, SDSS J0917+46, has Teff= 11,288 \pm 72 K
and log g = 5.48 \pm 0.03; with an estimated mass of 0.17 M_sun, it is the
lowest gravity/mass white dwarf currently known. However, 40 of the low mass
candidates are normal DA white dwarfs with apparently inaccurate SDSS g
magnitudes. We revisit the identification of low mass white dwarf candidates
previously found in the SDSS, and conclude that four objects have M < 0.2
M_sun. None of these white dwarfs show excess emission from a binary companion,
and radial velocity searches will be necessary to constrain the nature of the
unseen companions.Comment: ApJ, accepted versio
Detecting gravitational waves from precessing binaries of spinning compact objects. II. Search implementation for low-mass binaries
Detection template families (DTFs) are built to capture the essential
features of true gravitational waveforms using a small set of phenomenological
waveform parameters. Buonanno, Chen, and Vallisneri [Phys. Rev. D 67, 104025
(2003)] proposed the ``BCV2'' DTF to perform computationally efficient searches
for signals from precessing binaries of compact stellar objects. Here we test
the signal-matching performance of the BCV2 DTF for asymmetric--mass-ratio
binaries, and specifically for double--black-hole binaries with component
masses (m1,m2): (6~12Msun, 1~3Msun), and for black-hole--neutron-star binaries
with component masses (m1,m2) = (10Msun, 1.4Msun); we take all black holes to
be maximally spinning. We find a satisfactory signal-matching performance, with
fitting factors averaging between 0.94 and 0.98. We also scope out the region
of BCV2 parameters needed for a template-based search, we evaluate the template
match metric, we discuss a template-placement strategy, and we estimate the
number of templates needed for searches at the LIGO design sensitivity. In
addition, after gaining more insight in the dynamics of spin--orbit precession,
we propose a modification of the BCV2 DTF that is parametrized by physical
(rather than phenomenological) parameters. We test this modified ``BCV2P'' DTF
for the (10Msun, 1.4Msun) black-hole--neutron-star system, finding a
signal-matching performance comparable to the BCV2 DTF, and a reliable
parameter-estimation capability for target-binary quantities such as the chirp
mass and the opening angle (the angle between the black-hole spin and the
orbital angular momentum).Comment: 18 pages, 15 figure
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